1. Weathering and Soils Chapter 6

2. Earth’s surface processes
Weathering and Erosion –

3. Weathering Erosion involves the transport of rock fragments.
Weathering involves the physical breakdown and chemical alteration of rock at or near Earth’s surface
Two types:
Mechanical weathering – physical forces breaking rocks into smaller pieces
Chemical weathering – chemical transformation of rock into new compounds
Both types work simultaneously and reinforce each other
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4. Mechanical Weathering Increases Surface Area

5. Mechanical Weathering
Types of Mechanical Weathering
Frost wedging
Salt crystal growth
Sheeting/Unloading
Biological activity
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6. Frost Wedging Two different methods
Water works its way into cracks in rocks and the freezing enlarges the cracks in the rocks
Lenses of ice grow larger as they attract liquid water from surrounding areas
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7. Evidence of Frost Wedging in Wheeler Park, Nevada
Source: Tom Bean/DRK Photo

8. Ice Breaks Rock

9. Salt Crystal Growth
Sea spray or salty groundwater penetrates crevices and pore spaces in rocks
As the water evaporates, salt crystals form and enlarge the crevices
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10. Sheeting/Unloading/Exfoliation
Large masses of igneous rock are exposed by erosion and concentric slabs break loose
An exfoliation dome is formed after continued weathering causes the slabs to separate and spall off
Joints are fractures produced by contraction during the crystallization of magma
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12. Unloading - Exfoliation of a Pluton

13. Mechanical Exfoliation in Yosemite National Park
Source: Phil Degginger/Earth Scenes

14. Biological Activity
Plant roots grow into fractures in a rock, causing the cracks to expand
Burrowing animals break down rocks by moving fresh material to the surface, enhancing physical and chemical weathering
Human impacts (rock blasting) is very noticeable
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15. Types of Mechanical Weathering
Source: Tom Bean/DRK Photo

16. Chemical Weathering The Most Important Agent Is Water
Responsible for transport of ions and molecules involved in chemical processes
Types of Chemical Weathering
Dissolution
Oxidation
Hydrolysis
Spheroidal weathering
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17. Oxidation Oxygen combines with iron to form iron oxide
Process is slow in dry environments
Water increases the speed of the reaction
Important in decomposing ferromagnesium minerals like olivine, pyroxene, hornblende, and biotite
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19. Dissolution Certain minerals dissolve in water
Halite is one of the most water-soluble minerals
A small amount of acid in water increases the corrosive force of water, causing dissolution
Carbonic acid is created when carbon dioxide dissolves in raindrops
Calcite is easily attacked by weakly acidic solutions
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20. Hydrolysis The reaction of any substance with water
A hydrogen ion attacks and replaces another ion
Silicates primarily decompose by hydrolysis
Clay minerals are the most abundant product of weathering
Clay minerals are very stable under surface conditions
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21. Acid Rain

22. Spheroidal Weathering
Weathering attacks edges from two sides and corners from three sides
Sharp edges gradually wear down and become rounded
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23. Spheroidal or Joint-controlled weathering in igneous rocks
Definition: Joints vs Faults
Increases surface area
for chemical weathering

24. Three types of Chemical Weathering

25. Rate of Weathering The rate of weathering is influenced by:
Rock type
Dependent of mineralogy
Silicate minerals weather in the same order as crystallization (Bowen’s reaction series)
Climate
The rate of weathering is influenced by rock type (composition) and climate
Different minerals weather at different rates
Warm, moist climates enhance (and cold, dry climates inhibit) chemical weathering
Variations in local climate and the composition of the rock formation will produce uneven weathering of the rock called differential weathering
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26. Monuments to Weathering: An Example of Differential Weathering

27. Bowens Reaction Series and Weathering

29. Soil is “the bridge between life and the inanimate world” The bridge between the various Earth systems
Soil

30. Vegetation and Soil Development
Plants use Hydrolysis to get nutrient metals out of minerals

31. Climate also very important, see below
Factors related to bedrock composition (parent material)
Weathering-resistant sandstone (mostly quartz) yields little soil
Soil
Soil
Soil
Chemical
weathering
by hydrolysis
Iron-rich
basalt
Chemical
weathering
by oxidation
Feldspar-rich
granite
Chemical weathering
by dissolution
Limestone
Climate also very important, see below

32. Soil
Earth’s land surface is covered by a layer of rock and mineral fragments produced by weathering, called regolith
Soil is a combination of mineral and organic matter, water, and air and is the portion of the regolith that supports the growth of plants

33. Controls of Soil Formation
Parent material, time, climate, plants and animals, and topography interact to control soil formation

34. Controls of Soil Formation
Parent material
The source of weathered material that forms soil
Residual soils – soils form from the underlying bedrock
Transported soils – soils that form in place from unconsolidated sediment
Time
Weathering over a short period of time forms thin soils that closely resemble the parent material
Soils that have weathered for a long period of time are generally thick and do not resemble the parent material

35. Controls of Soil Formation
Climate
The most influential control of soil formation
Key factors are temperature and precipitation
Plants and Animals
Influence the soil chemistry
Remains are converted into humus which is an important part of the organic component of soils

36. Plants Influence Soil

37. Controls of Soil Formation
Topography
Steep slopes have poorly developed soils
Moisture content of these areas is often insufficient for plant growth
Flat and undulating surfaces are optimal for soil formation
Good drainage and minimal erosion
Slope orientation is also important in soil formation
Southern-facing slopes in the Northern Hemisphere receive the most sunlight are are optimal for soil formation

38. The Soil Profile
Soil-forming processes operate from the surface downward
Soil is divided into horizontal layers called horizons
A vertical section through all the soil horizons is called a soil profile
A mature soil has well-developed horizons
An immature soil may lack soil horizons

39. The Soil Profile O soil horizon—organic matter
The lower portion is composed of humus
This horizon includes bacteria, fungi, algae, and insects
A soil horizon—organic and mineral matter
High biological activity
O and A horizons make up the topsoil
E horizon—little organic matter
Light-colored layer
Eluviation (washing out fine soil components to lower soil layers) is common in this layer
Soluble inorganic components are washed to lower layers in a process called leaching
B horizon (subsoil)—zone of accumulation
Material washed down from the E horizon accumulates in this layer

40. Soil Horizons

41. Classifying Soils
Variations in soil formation over time and distances has led to a great variety of recognized soil types
Groups have been established using common characteristics
In the United States, a system was devised and called the Soil Taxonomy

42. Global Soil Regions

43. The Impact of Human Activity on Soils
Soil Erosion
Soil erosion is a natural process in the rock cycle
Erosion rates are dependent on climate, slope, and type of vegetation
Human activities such as deforestation and farming practices can enhance soil erosion